The river waters, the sea waters, the blood, etc. are the examples of a fluid, or a powdery fluid, consisting of a mixture of a plurality of solid components. These are the mixture of liquid components and solid components; although the solid components such as sand grains, germs, blood cells may precipitate or disperse in liquid component, they do not dissolve into the liquid component, but they keep on staying as the solid components.
The conventional method of separating it into a plurality of constituent components is described in the following, taking a blood cell/blood plasma separator as the example.
A blood sample is normally made available in the form of a whole blood which is consisting of blood plasma, or the liquid component, blood cells, or the solid component, and others. In most of the practical occasions for use as the test specimen, however, only the portion of blood cells, or the blood plasma, is needed.
In examining the blood sugar value of a blood, for example, the content of blood sugar dissolved in blood plasma is the subject of inspection; while for the detection of a DNA, it needs to be taken out of a white blood cell in the blood. It has been a conventional practice to put a sample of the whole blood in a test tube, and apply a centrifugal force. Each of constituent components of the whole blood in the test tube undergoes different centrifugal force, and the components are separated in accordance with the mass.
The blood plasma can be taken out when the supernatant liquid is extracted, while the blood cells can be taken out from the precipitate. The respective portions thus separated are used as the sample of examination for the specific purpose.
There is another method using a filter, for separating a sample of very small amount.
This is the method disclosed by Yong-Kyu Yoon, et. al. The method makes use of the porosity of a filter; where, blood plasma is made available by filtering blood cells larger than a certain specific size, or blood cells are taken out otherwise. The separation characteristic of the method depends on the size and the number of holes, etc. of a filter. Therefore, the design of a filter needs to be optimized for a targeted component of separation. A typical method for providing a mesh filter with a good reproducibility in terms of the size and numbers is exposing a photo-sensitive resist in a three dimensional arrangement.
However, the described previous method by means of centrifugal separation has the following problems. Namely, a sample of whole blood has be put into such a vessel as a test tube for the separation by centrifugal force; which means that a test tube needs to be filled with a certain amount of whole blood, viz. a sample of whole blood is needed for several to several tens of milliliters. Thus it is difficult to use the method for the samples of very small amount.
Meanwhile, the filtering method, which traps, or allows, solid grains of a certain size to go through when a fluid, or a powdery fluid, consisting of a mixture of a plurality of components is forced to pass through a filter, has a problem that the existence of solid particles of different sizes makes separation of components difficult. Namely, when a filter which has been designed to allow only small particles to go through is used in order to take out the particles of a certain specific size, the particles larger than the size of the small particles are trapped by the filter to cause a clogged filter. As the result, passing of the small particles is blocked.